The present invention relates in general to a method and apparatus for measuring uranium isotope enrichment (the uranium-235 content) in a uranium solution, and more particularly, although not limited, to a method and apparatus in which uranium isotope enrichment in processing steps is detected and supervised to provide criticality safety control, for example, in a uranium denitration system in spent nuclear fuel reprocessing facilities.
It is usualy important in uranium denitration system that the uranium isotope enrichment is supervised so that it stays at a constant value, or below it, thereby preventing the threat of a critical state arising. This supervision of the uranium isotope enrichment must be carried out quickly in view of the process of the system.
A mass spectrograph which is large in size and difficult to maintain is used conventionally for measuring the uranium isotope enrichment of a uranium solution, therefore it is inevitably necessary to provide pretreatment for applying a sample to be measured onto a special filament, a long measurement time, and the operation of special apparatus. The uranium isotope enrichment thus cannot be measured quickly and simply.
There is also a passive assay method of measuring uranium isotope enrichment which uses an X-ray tube or special radiation (ytterbium-169). However, in the method using the X-ray tube, a special, large high-voltage power unit is required for the X-ray tube, and also large quantities of unnecessary X-rays are produced by the X-ray tube so that a special energy filter is needed through which the required X-rays only are selectively extracted, and also a large shield is necessary. With the latter method using special radiation, a sealed active source is required, and the decay of the working source is quick (the half-life of ytterbium-169 is 32 days) and thus the apparatus cannot be maintained normally unless the radiation source is replaced about five times a year.
Such being the circumstances, neither method is applicable for the purpose of rapidly supervising the uranium isotope enrichment of a uranium solution particularly in the uranium denitration system described above.
Recently, however a method for solving these defects has been proposed (Japanese Patent Application Laid-Open No. 57-151881, laid-open Sept. 20, 1982), which comprises irradiating photons of an energy lower than the uranium K-edge energy and other photons of an energy higher than that energy onto a sample containing uranium to determine the concentration of uranium in the sample from the respective intensities of the photons which penetrate the sample, measuring an intensity of gamma rays emitted from the sample due to the alpha decay of uranium-235 in the sample, and obtaining a value of uranium isotope enrichment from the measured intensity of gamma rays and the concentration of uranium. An apparatus for practising the above-described method has also be proposed, which comprises a radiation source emitting photons of an energy lower than the uranium K-edge energy and other photons of an energy higher than that energy, a shutter capable of selectively shutting off the photons from the radiation source, a sample onto which the photons are irradiated through the shutter and through a collimator, a photon detector for detecting the intensity of the photons which penetrate the sample and the intensity of gamma rays emitted from the sample due to the alpha decay of uranium-235 in the sample, and a measuring electronic system including a pulse height analyzer for processing outputs of the photon detector.
This method and apparatus are advantageous for measuring uranium isotope enrichment quickly and easily, and is particularly effective for the purpose described above. However, no disclosure is made of a concrete example of the radiation source, and there are still problems remaining such that an apparatus which is compact and easy to maintain cannot be constructed from conventional prior art techniques, and that the measurement accuracy cannot be improved because of errors arising from the self-absorption of gamma rays emitted from uranium-235 in the sample.